The present invention relates to data communication systems which perform transmission and reception of digital data through duplex communication systems and more particularly. to a data communication system which provides optimum throughput in response to a fluctuation in the error rate on a packet radio channel.
Protocols for data transmission developed for TD MA (Time Division Multiple Access) based radio telephone systems typically use error control mechanisms like ARQ (Automatic Repeat Request) or a hybrid ARQ scheme supported by a FEC (Forward Error Correction) to reduce the error rate for the ARQ scheme.
A typical ARQ procedure for this function is the HDLC (High Level Data Link Protocol) procedure, defined by the International Telecommunication Union (ITU ) in ITU-T X.25.
Hybrid ARQ schemes are typically combined in that form, that FEC reduces the remaining error rate as a function in layer 1 according to the ISO-OSI reference model X 200! and ARQ as a layer 2 function. This system has to be adapted to the maximum expected remaining error rate. An example can be seen within the Radio Link Protocol (RLP), designed for circuit switched data transmission within the Global System for Mobile Communication GSM GSM 04.22!.
These prior art procedures do not achieve optimal throughput behaviour in low error rate channels due to the fixed channel coding, while reducing the basic error correction capability from the system would reduce the throughput in cases of high error rate channels.
Therefore stepwise combination of redundancy using hybrid type II ARQ/FEC schemes are proposed to get better throughput behaviour Man-delbaum 74!.
These prior art procedures are designed to be used on top of circuit switched duplex communication channels. They lose much of their efficiency if they are used on top of packet switched ratio channels due to the necessary channel reservation period for each positive or negative acknowledgement that has to be transmitted in case of errors.